Poly(vinyl fluoride) (PVF) is used in film form for a variety of protective and decorative applications. Because of its proven weatherability and moisture barrier properties, poly(vinyl fluoride) is particularly desirable for use as a protective layer in backsheets for photovoltaic modules.
The polymerization of vinyl fluoride (VF) to produce PVF has typically been carried out in water as the polymerization medium and by employing a water soluble organic azo-type initiator such as 2,2′-azobis(2-amidinopropane)dihydrochloride and 2,2′-azobis(N,N′-dimethyleneisobutyroamidine)dihydrochloride to yield a highly fluid aqueous dispersion or slurry of very finely divided polymer particles. Polymerizations of this type, in which the initiator is water soluble and the monomer has limited water solubility, are often termed emulsion polymerizations.
PVF has been produced in emulsion polymerization processes which do not employ surfactants. In processes of this type, significant limitations and problems have been encountered. In batch processes which produce an aqueous PVF dispersion, the PVF solids content has typically been limited to about 15 weight percent or less. If it is attempted to increase the solids content, coagulum (undispersed polymer which must be discarded as waste) results and/or undesirable particle aggregates form which can cause defects in film made from the dispersion. In continuous processes which produce a slurry of aggregates of primary PVF particles, a significant problem encountered is the broad particle size distribution of the polymer particles formed. It is desirable for the PVF to be produced as a slurry of aggregates of primary particles with the aggregates being in the size range of 6-11 μm which enables the PVF to be separated from the aqueous medium by filtration. However, in a continuous PVF polymerization process, a significant amount of larger particle aggregates are formed, some of which can be as large as 400 μm. Larger particles, such as those above 30 μm, produce defects in polyvinyl fluoride film formed from the PVF polymer resin and cause reactor scaling, requiring periodic shutdowns for reactor cleaning. To attempt to control particle size, the typical approach has been to limit the PVF solids content produced by the process. This necessarily limits the capacity of the reactor and has been found to be only partially successful in preventing the formation of large particles.
It is known in the art of emulsion polymerization that the addition of surfactants aids in stabilization of the polymer particles formed and can improve polymerization processes. For fluoropolymers which contain tetrafluoroethylene and/or other perfluorinated monomers, anionic fluorosurfactants such as ammonium perfluorooctanoate (APFO) have typically been used since they are generally non-telogenic and do not inhibit the polymerization reaction. For polyvinyl fluoride) polymerizations employing cationic organic azo-type initiators, an anionic fluorosurfactant cannot be used because it will form an insoluble complex salt with the initiator. U.S. Pat. No. 5,229,480 to Uschold discloses, for use in the polymerization of vinyl fluoride, a cationic fluorosurfactant of the formula F(CF2CF2)nRNH3X where R is selected from at least one of CH2CH2CH2 and CH3CCH3, n is an integer from 3-5, and X is an anion free from hydrogen atoms susceptible to attack by initiator or polymer radicals. However, because of environmental concerns which have been raised concerning APFO and because fluorosurfactants are expensive, a polymerization process employing a halogen-free surfactant would be desirable.
A variety of halogen-free nonionic and cationic surfactants are commercially available and are known to be useful in emulsion polymerization of hydrocarbon polymers, e.g., polystyrene, styrene-butadiene copolymers, polyvinyl acetate, etc. Examples of many such materials are described in D. C. Blackley, “Emulsion Polymerization”, Halsted Press, New York, N.Y., 1975, pp 308-318. The nonionic surfactants described by Blackley contain hydrophilic segments derived from oligomers of ethylene oxide. While these surfactants are compatible with the organic azo-type initiators, when used in the polymerization of vinyl fluoride in the amounts typically used in hydrocarbon polymer polymerizations, they produce low yields of low molecular weight PVF products that are not useful in typical applications for PVF. Common cationic surfactants such as dodecylammonium chloride, cetyltrimethylammonium bromide or cetylpyridinium bromide are stated by Blackley as being useful in hydrocarbon polymer polymerizations. While these surfactants are also compatible with organic azo-type initiators, U.S. Pat. No. 5,229,480 to Uschold shows that the use of surfactants of this type produce PVF of low molecular weight in reduced yields and often contribute a highly undesirable yellow color to the polymer.